making data files with gcc

[Christian Convey]

Hi Dale,

If you want to produce data files that have certain content and structure, the 
traditional (and usually best) approach is to have a program that contains code 
for writing (and perhaps reading) such files.

In the example code you supplied, the most straight-forward approach to your 
probem would be to write a function (or perhaps a method on the gFast struct) 
that can write the struct's fields to a file (or more generally, some output 
stream).  For example:

// Returns 'true' if and only if the write operation was successful
bool write_gFast(ostream & s, const gFast & g) {
     s << g.entries << g.size << g.cnt;

     int bytesOfData = g.entries * g.size; // Is this formula right?
     bool success = s.write(g.data, bytesOfData);
     return success;

     // Note: I *think* this is an ok way to check the status of a write
     // operation, but you should double-checked the C++ documentation.
}


// Returns 'true' if and only if the read operation was successful
bool read_gFast(istream & s, const gFast & g) {
     s >> g.entries >> g.size >> g.cnt;

     int bytesOfData = g.entries * g.size; // Is this formula right?
     g.data = new char[bytesOfData];
     // (You should somehow ensure that the "new" command didn't return NULL)

     bool success = s.read(g.data, bytesOfData);
     return success;

     // Note: I *think* this is an ok way to check the status of a read
     // operation, but you should double-checked the C++ documentation.
}


Then your main code might look like this. (You should double check the C++ docs 
on the header files needed for ifstream and ofstream.)

#include <iostream>
#include <fstream>
using namespace std;

...

// To write a data file. (Assumes you already have a gFast instance set up
// with the values you want to appear in the file...

ofstream outFile("someNewFile", "w");
bool success = write_gFast(outFile, someGfastInstance);

HTH,
Christian

Dale Walsh wrote:
> I'm trying to generate data files however it seems to be prepending  244 
> bytes of data and I can't see how to prevent this.
> 
> The actual data files are very large and manually using a hex editor  to 
> generate them isn't appealing considering there are about 300 of  them.
> 
> 
> 
> Here is my test compile session with a reduced file with 2 rows of 10  
> bytes so the file should be 32 bytes in length.
> 
> test.c: (combined header and source)
> ___________________________
> 
>   typedef struct
>   {
>     /* # of entries in data */
>     int entries;
>     /* entry size and count */
>     int size;
>     int cnt;
>     /* data; array of bytes, one row after another. */
>     char *data;
>   }
>   gFast;
> 
> /* Location functions take these. */
>   typedef gFast * gFastPtr;
> 
> char TestDATA[20];
> 
> gFast gFastTestRep = {
>     2,
>     2,
>     10,
>     TestDATA
> };
> 
> gFastPtr gFastTest = &gFastTestRep;
> /*
>    this is all the data
>    the file should be 32 bytes in length
>   20 bytes of data and 12 bytes for header
> */
> 
> char TestDATA[] = {
>     
>     /* Entry 000 */
>     1,2,3,4,5,6,7,8,9,10,
> 
>     /* Entry 001 */
>     11,12,13,14,15,16,17,18,19,20,
> };
> 
> ___________________________
> 
> 
> compiled with:
> gcc -pipe -static -c -o test.out test.c
> 
> ___________________________
> 
> 
> Here's the resulting file content, there are always 244 leading bytes  
> of data that I can't have, the stuff appended to the end would also  be 
> nice to not have however it wont interfere with the file being  valid to 
> the application and isn't as critical.
> 
> mustangrestomods:/temp2/clamav-0.80 root# hexdump -vC test.out
> 00000000  fe ed fa ce 00 00 00 12  00 00 00 00 00 00 00 01   
> |................|
> 00000010  00 00 00 02 00 00 00 d8  00 00 20 00 00 00 00 01   |.......... 
> .....|
> 00000020  00 00 00 c0 00 00 00 00  00 00 00 00 00 00 00 00   
> |................|
> 00000030  00 00 00 00 00 00 00 00  00 00 00 28 00 00 00 f4   
> |...........(....|
> 00000040  00 00 00 28 00 00 00 07  00 00 00 07 00 00 00 02  |... 
> (............|
> 00000050  00 00 00 00 5f 5f 74 65  78 74 00 00 00 00 00 00   
> |....__text......|
> 00000060  00 00 00 00 5f 5f 54 45  58 54 00 00 00 00 00 00   
> |....__TEXT......|
> 00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 f4   
> |................|
> 00000080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   
> |................|
> 00000090  00 00 00 00 00 00 00 00  5f 5f 64 61 74 61 00 00   
> |........__data..|
> 000000a0  00 00 00 00 00 00 00 00  5f 5f 44 41 54 41 00 00   
> |........__DATA..|
> 000000b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 28   
> |...............(|
> 000000c0  00 00 00 f4 00 00 00 02  00 00 01 1c 00 00 00 02   
> |................|
> 000000d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 02   
> |................|
> 000000e0  00 00 00 18 00 00 01 2c  00 00 00 03 00 00 01 50   
> |.......,.......P|
> 000000f0  00 00 00 24 00 00 00 02  00 00 00 02 00 00 00 0a  |... 
> $............|
> 00000100  00 00 00 14 00 00 00 00  01 02 03 04 05 06 07 08   
> |................|
> 00000110  09 0a 0b 0c 0d 0e 0f 10  11 12 13 14 00 00 00 10   
> |................|
> 00000120  00 00 02 40 00 00 00 0c  00 00 02 40 00 00 00 0f   
> |...@.......@....|
> 00000130  0f 02 00 00 00 00 00 14  00 00 00 19 0f 02 00 00   
> |................|
> 00000140  00 00 00 10 00 00 00 01  0f 02 00 00 00 00 00 00   
> |................|
> 00000150  00 5f 67 46 61 73 74 54  65 73 74 52 65 70 00 5f   
> |._gFastTestRep._|
> 00000160  54 65 73 74 44 41 54 41  00 5f 67 46 61 73 74 54  | 
> TestDATA._gFastT|
> 00000170  65 73 74 00                                       |est.|
> 00000174
> 
> 
> 
> 
> -- Dale
> 

-- 
Christian Convey
Computer Scientist,
Naval Undersea Warfare Centers
Newport, RI